1. Field of the Invention
The present invention relates to an electronic still imaging apparatus using a solid state imaging device such as a charge coupled device (CCD), etc., and more particularly to an electronic still imaging apparatus for executing dynamic rage expanded imaging for forming a single image by synthesizing a plurality of original images picked up in a different exposure amount, to an electronic still imaging apparatus and an electronic still imaging method having a function for acquiring a synthesis image having a wide dynamic range by synthesizing image signals of a plurality of images each having a different exposure amount and output from a solid state imaging device.
2. Description of the Related Art
Many trials have been proposed to execute dynamic-range-expanded imaging using an electronic still imaging apparatus, and, for example, an imaging system for synthesizing at least two original images acquired by being picked up in a different exposure amount has been known.
When a camera shake arises in imaging at a time this imaging system is employed, an image becomes blurred between timings at which an original image is picked up twice, thereby each pixel does not correspond to the same point of an object.
Accordingly, the image quality of a synthesis is collapsed, that is, it is extremely deteriorated to an unacceptable level.
Jpn. Pat. Appln. KOKAI Publication No. 2002-165138 discloses a technology for avoiding collapse of image quality by employing an imaging sequence in which two images are picked up particularly at continuous timing as well as by permitting to use dynamic-range-expanded imaging only when a shutter speed shorter than a predetermined camera shake limit shutter speed can be used.
Although this technology is very effective to avoid the collapse of the image quality, application of the dynamic-range-expanded imaging is limited only to an object having high luminance to which a high speed shutter can be used.
In general, in an electronic camera using a CCD imaging device, when an image is picked up using an AE exposure (preliminary imaging) and recorded, a pickup image signal is converted into digital image data by an A/D converter.
When it is assumed that the A/D converter has 10 conversion bits, the output value of the image data is located within the range of 0 to 1023.
Of the image data within this range, image data whose value is 32 or less on a dark side, for example, is less reliable by affecting on a noise.
Further, since it is contemplated that image data is partly saturated on a bright side, image data having a value up to about 960 can be used.
Accordingly, brightness that can be recognized within the usable data range of 32 to 960 is about 30 times (960/32≈30), and this brightness is expressed by about five stages of Bv (luminance) value.
In contrast, since an electronic camera ordinarily has 16 stages of exposure possible ranges Bv(−3) to Bv(13), it can be found that the camera cannot cover the entire extent of brightness when the AE exposure is executed once because the number of conversion bits of the A/D converter is ordinarily 10 bits.
Therefore, when a range covered by an exposure executed once is overlapped to realize the AE exposure in correspondence to the entire extent of brightness in the exposure possible range of the camera, the AE exposure must be executed about four times as shown by exposures A, B, C, D in FIG. 14.
The exposure conditions of the respective exposures A, B, C, D are ordinarily adjusted by an electronic shutter of a CCD imaging device and a gain of a CDS circuit.
FIG. 15 is a table showing examples of an exposure condition to the medians (11, 7, 3, −1) of the respective luminance values Bv of the exposures A, B, C, D of FIG. 14.
Here, Av shows an aperture value, Tv shows a shutter speed, and Sv shows sensitivity, respectively.
Note that the Av, Tv, and Sv values corresponding to the Bv value are calculated from a known program diagram as shown in FIG. 17.
The AE exposure condition of a final imaging is calculated from the image data acquired from the AE exposure executed four times as described above, and a method of calculating the exposure condition will be described below.
First, as shown in FIG. 16, a specific region within an imaging angle of view, from which an edge portion is removed, of a pickup image acquired by preliminary imaging executed four times is divided into 9×9 blocks, the average values of RGB signals in each divided block are calculated, and luminance Y′ is calculated from the average values based on the following expression (1).Y′=0.299R+0.587G+0.114B  (1)
A weighed average value Y is calculated by multiplying the luminance Y′ by weighing coefficients (1 or 2) shown in the respective divided blocks shown in FIG. 16.
Then, the weighed average value Y is calculated as to the respective image signals acquired by the respective exposures A, B, C, D, and an AE exposure (preliminary imaging), which has a weighed average value within the range of 32-960, is selected from the four exposures A, B, C, D.
For example, it is assumed that the exposure C is selected and that Y=500.
At this time, the Bv value of an object is as shown in the following expression (2).Bv=3(aimed Bv value of exposure C)+log2[500/200(AE target value)]=4.3  (2)
The expression (2) means that when an object having brightness of the Bv value (3) is picked up with the exposure C, Y is set to 200 and that a shift amount from Bv value(3) is determined from the ratio of the calculated weighed average value Y of luminance and 200 (AE target value).
The Tv and Av values are determined from the Bv value determined as described above using the program diagram shown in FIG. 17.
For example, when the intersection P of the vertical line of a Bv value 4.3 and the sensitivity line of an Sv value 4 (ISO: 50) is determined and the intersection Q of a slant line passing through the intersection P and the program block is determined, and the aperture value Av and the shutter speed Tv at the intersection Q show the exposure condition in this imaging.
Incidentally, a solid state imaging device such as a CCD imaging device, and the like is ordinarily used in imaging apparatuses such as TV cameras, video cameras, digital cameras, and the like.
However, the solid state imaging device is disadvantageous in that the dynamic range thereof is very narrow as compared with that of a silver salt photographic film.
To solve this drawback, there has been proposed a method of acquiring an image having a dynamic range expanded by reading out image signals of two images having a different exposure amount from a single imaging device and synthesizing them.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2000-92378 proposes an imaging apparatus having a function of creating a synthesis image having a wide dynamic range by creating image signals of a plurality of images of the same object, the images each having a different exposure amount, that is, a long time exposure and a short time exposure and by subjecting the image signals to synthesis processing, wherein imaging can be executed by selecting one imaging mode from at least two imaging modes of an ordinary imaging mode, a forcible wide dynamic range imaging mode, and an automatic wide dynamic range imaging mode.
In an ordinary single exposure imaging, when light measuring processing is executed by executing a preliminary imaging exposure four times in order to determine a proper exposure condition, a predetermined time is taken from a time a shutter is pressed to a time a final exposure is executed, from which a considerably long time lag of the shutter arises.
A similar shutter time lag is also arisen in the imaging apparatus having the function for acquiring the image having the expanded dynamic range by reading out the imaging signals of the plurality of images having the different exposure amount from the single imaging device and by synthesizing the imaging signals, when light measuring processing is executed by executing a preliminary imaging exposure four times as in the imaging executed in the single exposure imaging in order to determine a proper exposure condition when exposures are executed four times under a different exposure condition or to set the number of times of exposures.